1. Field of the Invention
This invention relates generally to methods and apparatus for producing plasma; in particular, the invention relates to establishing a steady-state glow-discharge plasma at atmospheric pressure and low temperatures.
2. Description of the Related Art
Plasma is an ionized form of gas that can be obtained by ionizing a gas or liquid medium using an AC or DC power source. A plasma, commonly referred to as the fourth state of matter, is an ensemble of randomly moving charged particles with sufficient density to remain, on average, electrically neutral. Plasmas are used in very diverse processing applications, ranging from the manufacture of integrated circuits for the microelectronics industry, to the treatment of fabric and the destruction of toxic wastes.
In particular, plasmas are widely used for the treatment of organic and inorganic surfaces to promote adhesion between various materials. For example, polymers that have chemically inert surfaces with low surface energies do not allow good bonding with coatings and adhesives. Thus, these surfaces need to be treated in some way, such as by chemical treatment, corona treatment, flame treatment, and vacuum plasma treatment, to make them receptive to bonding with other substrates, coatings, adhesives and printing inks. Corona discharge, physical sputtering, plasma etching, reactive ion etching, sputter deposition, plasma-enhanced chemical vapor deposition, ashing, ion plating, reactive sputter deposition, and a range of ion beam-based techniques, all rely on the formation and properties of plasmas.
Corona discharges are widely used in particular for treating plastic films, foils, papers, etc. to promote adhesion with other materials by increasing the surface energy of the film. A corona discharge is established between two electrodes by applying a high voltage to one of the electrodes while the other is connected to ground at typical frequencies in the order of 10-50 kHz. These conditions produce locally concentrated discharges known in the art as streamers, which lead to some non-uniformity in the treatment of film surfaces and can also damage the film by producing low molecular weight species that adversely affect adhesion to the surface. Furthermore, the streamers of corona treatment can produce backside effects on the film being treated, which is undesirable in many applications. Nevertheless, corona treatment is extensively used in the industry for improving the surface energy of materials.
Glow-discharge plasma treatment is also an effective method of treating surfaces to increase their wettability and adhesion to various materials. Glow discharge provides a more uniform and homogenous plasma that produces a more consistent surface treatment than corona treatment, thereby avoiding unintentional back treatment of the film. Glow-discharge plasma is characterized by high-energy electrons that collide with, dissociate and ionize low-temperature neutrals, creating highly reactive free radicals and ions. These reactive species enable many chemical processes to occur with otherwise unreactive low-temperature feed stock and substrates. Based on these properties, low-density glow-discharge plasmas are usually utilized for low material-throughput processes involving surface modification. These plasmas are typically formed by partially ionizing a gas at a pressure well below atmosphere. For the most part, these plasmas are weakly ionized, with an ionization fraction of 10.sup.-5 to 10.sup.-1, established with AC or DC power in systems with varied geometries. These systems always require vacuum chambers and pumps to maintain a low pressure, which increases operating costs and maintenance.
There has been an extensive effort to develop plasma systems capable of operating at atmospheric pressure for surface treatment of polymer films, foils, and paper, in order to avoid capital and maintenance expenditures for vacuum chambers and pumps. It is known that atmospheric plasma can be generated at relatively low temperatures with a proper power source, the insertion of a dielectric layer between the electrodes, and the use of an appropriate gas mixture as plasma medium. For surface treatment of polymer films, fabrics, paper, etc., atmospheric plasma can be established between two electrodes using an inert gas such as helium under particular operating conditions. Usually one electrode is attached to a high voltage power supply, and a rotating drum is grounded and acts as the other electrode. One electrode is coated with a ceramic layer and the plasma gas is injected between electrodes.
For example, U.S. Pat. No. 5,456,972 describes a glow-discharge plasma system operating at atmospheric pressure. The apparatus consists of a pair of spaced plate electrodes energized over a range of about 1 to 5 KV rms at a radio frequency of about 1 to 100 KHz. A plasma gas is injected between the plates and the film to be treated is passed through them and exposed to the resulting plasma discharge for a predetermined period of time.
U.S. Pat. No., 5,789,145 discloses an atmospheric-pressure glow-discharge system based on conventional corona discharge apparatus. The improvement consists of pumping a gas comprising mostly helium through a gas delivery system containing slits that distribute the flow uniformly to the electrode arrangement. Various other patents describe particular gas compositions that enable the production of atmospheric or near-atmospheric pressure glow-discharge plasma for particular applications. See, for example, U.S. Pat. Nos. 5,387,842, 5,403,453, 5,414,324, 5,558,843, 5,669,583, 5,767,469, and 5,789,145.
In an attempt to improve the uniformity of the plasma created at atmospheric pressure and avoid the formation of streamers typical of corona treatment, perforated electrodes and screens have also been used. For example, U.S. Pat. No. 5,714,308 teaches a method for establishing a uniform atmospheric plasma between two electrodes using an inert gas or a mixture of gases as plasma medium. One of the electrodes is connected to an AC power supply. The other electrode is covered with an insulating material. As is, such an electrode arrangement will produce a corona discharge if powered by a voltage at 40-500 kHz. The invention consists of providing uniformly spaced holes in one electrode with a diameter of the order of 1 mm and pumping a gas through the holes, which has been found to produce atmospheric glow-discharge plasma at these frequencies using mixtures of helium and other gases, such as N.sub.2 and O.sub.2, so long as such other gases do not exceed 8.0%.
Thus, it has been shown that a uniformly perforated structure placed between electrodes improves gas diffusion and, correspondingly, makes it possible to obtain some level of glow-discharge plasma at higher pressures than previously possible. Other than under very specific operating conditions and limited gas compositions, though, these system cannot produce a uniform glow discharge at atmospheric pressure.
Therefore, there is still a need for a plasma treatment system capable of producing a steady glow discharge at atmospheric pressure with different gas mixtures. In addition, it would be desirable to be able to operate such a system at frequencies as low as 60 Hz.